A process and an installation of this kind are described in DE-OS 38 01 315. This publication describes the problems encountered when trying to set the water content of the wet plaster-fiber mixture and the possible solutions which have already been found. In the process described in DE-OS 38 01 315 the wetting of the mixture is carried out in two steps. A first portion of the required water is mixed into the dry mixture; the still spreadable mixture is then molded to a mat in several layers, similar to the process of producing pressed particle boards. The balance of the required water is then sprayed onto the individual layers of the spread mixture. The mat produced this way is then compressed into a raw plate in a press. The strength of the plates produced this way is very good, however it has been found that further improvement is possible.